Hydrogen chloride occurs as a by-product in the production of a plurality of chemical reactions with chlorine and or phosgene, for example the production of isocyanates or the chlorination of aromatic compounds. Hydrogen chloride can be converted by electrolysis or by oxidation with oxygen to chlorine, which can then be used again in such chemical reactions. The oxidation of hydrogen chloride (HCl) to chlorine (Cl2) takes place by reaction of hydrogen chloride and oxygen (O2) according to the following equation:4HCl+O22Cl2+2H2O
The reaction can be carried out in the presence of catalysts at a temperature of approximately 250 to 450° C. The normal reaction pressure is in the range of 1 to 10 bar. Such processes, generally referred to collectively as “Deacon processes”, are known, and include: Shell-Chlor process, MT-Chlor process, KEL-Chlor process, Carrier Catalyst process and Sumitomo-Chlor process.
Suitable catalysts for a Deacon process include transition metal compounds such as copper and ruthenium compounds or even compounds of other noble metals such as gold and palladium. Such catalysts are described for example in DE 1567788 A1, EP 251731 A2, EP 936184 A2, EP 761593 A1, EP 711599 A1 and DE 10250131 A1, the entire contents of each of which are hereby incorporated herein by reference. Such catalysts can normally be applied to a support material. Suitable support materials include, for example, silicon dioxide, aluminium oxide, titanium dioxide and/or zirconium oxide.
The Deacon processes are regularly carried out in fluid-bed reactors or fixed-bed reactors, e.g., in multi-tube reactors. Hydrogen chloride is freed of impurities before the reaction with oxygen in order to avoid poisoning of the catalysts used.
Alternatively, processes in which the reaction of hydrogen chloride with oxygen is non-thermally activated, are known. Such processes are described in the literature. “Non-thermally activated” reactions include, but are not limited to, excitations of the reaction with, for example, any one or more of the following: energy radiation, e.g., laser radiation, photochemical radiation sources, UV radiation, infrared radiation, etc.; a low-temperature plasma, e.g., created by electrical discharge; magnetic field excitation; tribomechanical activation, e.g., excitation by shock waves; ionising radiation, e.g., gamma-ray and X-ray radiation, α- and β-rays from nuclear disintegration, high-energy electrons, protons, neutrons and heavy ions; and microwave irradiation.
Oxygen is normally used in both the thermal and non-thermal activated reaction of hydrogen chloride as a pure gas with an O2 content of >98 vol. %.
The reaction of hydrogen chloride with oxygen is known to produce a gas mixture which, in addition to the target product chlorine, can also contains water, unreacted hydrogen chloride and unreacted oxygen, along with other secondary constituents such as carbon dioxide. To obtain pure chlorine, the product gas mixture is cooled until reaction water and hydrogen chloride condense out in the form of concentrated hydrochloric acid. The hydrochloric acid formed is then separated from the remaining gaseous reaction mixture. Residual water can then be removed from the gas mixture by washing with sulfuric acid or other methods such as drying with zeolites.
The hydrochloric acid separated from the gas mixture is normally then fed to a desorption stage in which gaseous hydrogen chloride is again released. This gaseous hydrogen chloride can be partially or preferably completely returned to the reaction of hydrogen chloride with oxygen. The dilute hydrochloric acid occurring in the desorption stage can be returned to the hydrochloric acid condensation stage. Here the dilute hydrochloric acid serves as an absorbing agent for the gaseous hydrogen chloride to be separated off. A procedure of this type is generally described, for example, in DE 10235476 A1. Alternatively, the hydrochloric acid separated from the gas mixture can also be fed to recycling.
The chlorine-containing reaction gas mixture freed of residual water is then compressed, wherein the unreacted oxygen and other secondary gas constituents can remain in the gas phase, and thus be separated from the liquefied (compressed) chlorine. Processes of this type for obtaining pure chlorine from gas mixtures are described for example in DE 19535716 A1 and DE 10235476 A1, the entire contents of each of which are hereby incorporated herein by reference. The purified chlorine can then be fed, for example, to a process for the production of isocyanates.
A substantial disadvantage of these processes is the relatively high expenditure of energy required for the desorption of hydrogen chloride from the hydrochloric acid during the purification of the chlorine gas stream after the reaction. Additionally, the recycling of the hydrochloric acid in such processes is not generally economic.